Silicon carbide (SiC) is a high-hardness semiconductor material with a greater bandgap than silicon (Si), and has been used extensively in various types of semiconductor devices including power elements, hostile-environment elements, high temperature operating elements, and radio frequency elements. Among other things, the application of SiC to power elements with switching and rectifying functions has attracted a lot of attention. This is because a power element that uses SiC can significantly reduce the power loss compared to a Si power element. In addition, by utilizing such properties, SiC power elements can form a smaller semiconductor device than Si power elements.
A metal-insulator-semiconductor field effect transistor (MOSFET) is a typical semiconductor element among various power elements that use SiC. And a metal-oxide-semiconductor field effect transistor (MOSFET) is one of those MISFETs.
Hereinafter, a general structure for a power element that uses SiC will be described with a MOSFET taken as an example with reference to the accompanying drawings.
FIG. 12(a) is a plan view generally illustrating a semiconductor element 1000, which is mostly made of a silicon carbide (SiC) semiconductor. The semiconductor element 1000 has a unit cell region 1000ul with an element function (which may be a switching function in the case of a transistor or a rectifying function in the case of a diode) and a terminal region 1000f which complements the breakdown voltage of the region with the element function. In the unit cell region 1000ul, arranged are a number of unit cells. In the example illustrated in FIG. 12(a), the terminal region 1000f is arranged around the unit cell region 1000ul. In forming MISFETs, source and gate electrodes of unit cells (to be described later) are connected in parallel with each other and gate pads to supply an electrical signal to the semiconductor element 1000 and a source pad to make current flow through the semiconductor element 1000 are arranged (none of which are shown in FIG. 12(a)) in the unit cell region 1000ul. FIG. 12(b) is a cross-sectional view illustrating one of the unit cells that are arranged in the unit cell region 1000ul. 
The unit cell 1000u includes an n-type semiconductor substrate (e.g., SiC substrate) 1010 with low resistivity, a silicon carbide semiconductor layer 1020 that has been formed on the principal surface of the semiconductor substrate 1010, a channel layer 1060 arranged on the silicon carbide semiconductor layer 1020, a gate electrode 1080 that is arranged over the channel layer 1060 with a gate insulating film 1070 interposed between them, a source electrode 1090 that contacts with the surface of the silicon carbide semiconductor layer 1020, and drain electrode 1100 arranged on the back surface of the semiconductor substrate 1010.
The silicon carbide semiconductor layer 1020 has a body region 1030, of which the conductivity type (i.e., p-type in this example) is different from that of the SiC substrate 1010, and a drift region 1020d, which is the rest of the silicon carbide semiconductor layer 1020 other than the body region 1030. More specifically, the drift region 1020d is an n−-type silicon carbide semiconductor layer including an n-type impurity, of which the concentration is lower than in the SiC substrate 1010. Inside the body region 1030, defined are an n-type source region 1040 including an n-type impurity at a high concentration and a p+-type contact region 1050 that includes a p-type impurity at a higher concentration than the body region 103.
The source region 1040 and the drift region 1020d are connected together through the channel layer 1060. By applying a voltage to the gate electrode 1080, a channel has been produced in a portion of the channel layer 1060 that contacts with the upper surface of the body region 1030.
Also, the contact region 1050 and the source region 1040 make ohmic contact with the source electrode 1090. Consequently, the body region 1030 is electrically connected to the source electrode 1090 via the contact region 1050.
As the unit cell 1000u has a pn junction between the body region 1030 and the drift region 1020d, the unit cell 1000u has a breakdown voltage of several hundred to several thousand volts (e.g., approximately 600 V to 10 kV) when a voltage that is positive with respect to the drain electrode 1100 is applied to the source electrode 1090. However, the electric field could be overconcentrated around the unit cell region 1000ul and the designed breakdown voltage could not be achieved. That is why in a general power element, some breakdown voltage compensating structure is usually provided for the terminal region 1000f. For example, a structure such as a field limiting ring (FLR), a junction termination edge or extension (JTE) or a resurf is provided for the terminal region 1000f (see Patent Documents Nos. 1 to 3).
FIG. 12(c) is a cross-sectional view of the terminal region 1000f, for which an FLR structure is provided as the terminal structure, as viewed on the plane E-F in the plan view shown in FIG. 12(a).
In this terminal region 1000f, a number of p-type field limiting ring (FLR) regions 1030f are arranged in an upper part of the silicon carbide semiconductor layer 1020. In the example illustrated in FIG. 12(c), each of those ring regions 1030f surrounds the unit cell region 1000ul in a ring. These ring regions 1030f can avoid overconcentration of an electric field in the unit cell region 1000ul and can minimize a decrease in breakdown voltage.
In some cases, a diode region 1150d may be provided between the unit cell region 1000ul and the terminal region 1000f. In the diode region 1150d, a p-type region 1030d is arranged in the silicon carbide semiconductor layer 1020. The p-type region 1030d and the n−-type drift region 1020d form a pn junction. In this description, such a breakdown voltage compensating structure, including the ring region 1030f and the diode region 1150d, will be referred to herein as a “terminal structure”.
The ring regions 1030f are usually formed by implanting ions of a p-type impurity into the silicon carbide semiconductor layer 1020. In a power element that uses silicon carbide, either Al ions or B ions may be used as ions of a p-type impurity. In that case, the ion implantation condition is set so that the ring regions 1030f have as uniform an impurity concentration profile in the depth direction as possible.
Meanwhile, Patent Documents Nos. 4 and 5 disclose arranging girdling, which is designed so as to have a certain concentration difference, in the terminal region.